FIG. 42 illustrates prior art transmission lines for a microwave/millimeter wave integrated circuit. In the figure, a microstrip line which is generally used is illustrated as an example. In FIG. 42, reference numeral 1 designates a dielectric or a semi-insulating semiconductor substrate, reference numeral 2 designates a grounded conductor formed on the rear surface of the substrate 1, and reference numeral 3 designates a microstrip conductor. The transmission line fabricated thereby has the advantage of easy fabrication. However, since it has an open structure, its confinement of an electromagnetic wave becomes poor.
FIGS. 43(a)-43(b) are schematic views describing difficulties concerning the transmission line of the prior art microwave/millimeter wave integrated circuit. The arrows in the figures represent electric field lines. As illustrated in FIG. 43(a), it is known that, if a plurality of lines are formed in close proximity of each other, electromagnetic wave interference between the lines, also known as crosstalk, occurs. Therefore, the lines of the microwave/millimeter wave integrated circuit are generally arranged with sufficient distance between the lines 3 as illustrated in FIG. 43(b).
Furthermore, it is known that a microstrip line produces higher order modes of propagation of an electromagnetic wave in an ultra high frequency band. In order to avoid the occurrence of these higher order modes of electromagnetic wave propagation, it is necessary to reduce the thickness of the substrate and the width of the strip conductor. However, in the case of a GaAs substrate, the thickness of the substrate has to be 30 .mu.m or less in a 500 GHz band. The production of a substrate whose thickness is 30 .mu.m or less is extremely difficult, and even if it is produced, the reliability of the integrated circuit is considerably reduced.
Since a prior art transmission line for a microwave/millimeter wave integrated circuit is structured as described above, the following difficulties arise.
1. If a plurality of lines are in close proximity of each other, electromagnetic wave interference, i.e., crosstalk, results. In order to avoid it, the chip size must increase and the production cost rises.
2. In order to avoid the occurrence of higher order modes of electromagnetic wave propagation in a transmission line for an ultra high frequency band, it is necessary to significantly reduce the thickness of the substrate, and this considerably deteriorates the production yield and the reliability of the integrated circuit.